1. Field of the Invention
This invention generally relates to a zoom lens, and, in particular, to a ultra compact zoom lens whose distance between the front surface of its front lens and an image point is extremely small.
2. Description of the Prior Art
Recently, even in a compact camera using a lens shutter, a bifocal camera capable of switching between the quasi-wide angle mode and the telepscopic mode has become a main stream. Besides, there is a growing demand for a compact camera equipped with a zoom lens which allows to change the focal distance continuously and not just between the two fixed focal distances as in the bifocal camera. Moreover, although a zoom lens has been used with a video camera, there has been a new demand for a zoom lens more compact in size because of the advent of an 8-mm video camera. In order to meet with these new demands, there must be provided a zoom lens which does not have a relatively long back focal length, which was usually the case with the prior art single reflex cameras, and whose distance between the front surface of the front lens and an image point is extremely small.
Under the circumstances, the present inventor has previously proposed an ultra compact zoom lens including first, second and third lens units having positive, negative and positive refracting powers, respectively, from the front to the rear with the paraxial lateral magnification of the second lens unit being always larger than unity and the third lens unit being fixed in position, whereby the magnification is varied by varying the distances among the lens units by moving the first and second lens units, as disclosed in the Japanese Patent Application No. 61-28890 assigned to the assignee of this application, which is hereby incorporated by reference. This proposed zoom lens has the telephoto ratio in the order of 1.2 to 1.07 and the diameter of its front lens is relatively smaller, thereby allowing to make the entire structure compact in size. In addition, despite its compactness, it has an excellent image forming characteristic over the entire zooming range. However, the zoom ratio cannot be increased beyond approximately 1.7.